Radio communication devices use antennas to provide for the efficient transmission of radio frequency (RF) communication signals. The transmitter portion of a radio communication device includes a power amplifier to amplify the RF signals before they are coupled to the antenna for transmission. For some modulation techniques, linear amplification is desired to prevent distortion of the modulated signal. However, when RF power amplifiers are operated in their most efficient manner at high drive levels, they usually provide a nonlinear “compression” characteristic. This means that a change in the amplitude of a signal sent into the power amplifier results in a non-proportional change in the amplitude of the signal out of the amplifier, and therefore causes distortion of the signal.
One manner of improving the linearity of a RF transmitter is to use a Cartesian feedback system, whereby a feedback signal path is provided to create a negative feedback within the transmitter which compensates for the compression in the power amplifier. However, the stability associated with Cartesian feedback is influenced by outside factors, such as the reflected signals at the antenna and temperature drift.
In some Cartesian feedback systems, the base station interrupts normal communication traffic and uses half a slot to linearize or train itself. What is transmitted is typically left up to the manufacturer to define. The utilization of a training slot can be intrusive to others and lowers the useful throughput if done too often.
Multicarrier systems present additional design challenges as Cartesian feedback incurs additional delays making the ability to maintain stability difficult. If several carriers are transmitted through the same transmitter the carriers have to be synchronized so that signals are not destroyed. This synchronization might not be possible if signals are carrying modulation having different protocols. Factors like thermal drift may make it difficult to maintain stability.
Accordingly, it would be highly desirable to have an improved means of operating a transmitter.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.